In a Premixed Compression Ignition (PCI) engine or Homogenous Charge Compression Ignition (HCCI) engine, fuel is introduced into the cylinder prior to or early during the compression stroke, i.e. prior to movement of the piston towards top dead center (TDC). As the piston moves towards TDC during the compression stroke, the air and fuel mixture is compressed. As a consequence, the temperature and pressure within the cylinder increases continuously during the compression stroke. The increasing temperature results in auto ignition of the air and fuel mixture at crank angles at or near TDC. The result is combustion characterized by a rapid cascade of detonations in different zones of the cylinder as the piston is approaching TDC. This allows the realization of an effective expansion ratio approaching the geometric compression ratio (i.e, maximum efficiency) of the engine. Additionally, if the air and fuel mixture is sufficiently lean, the engine-out NOx and soot emissions will be dramatically lower than those of typical Diesel and SI engines.
Most HCCI engines lack an actuating mechanism capable of directly regulating the start of combustion and the rate of heat release in combustion. Instead, combustion begins when the air and fuel mixture has been compressed to sufficiently high pressures and temperatures, and the progression of combustion is primarily determined by the chemical kinetics of the combustion reaction